import java.util.Scanner;

//找规律做的题
public class Main{
    
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int row = sc.nextInt(), col = sc.nextInt();
        int[][] array = new int[row][col];
        
        for(int i = 0; i < row; i++){
            if(i % 4 == 0 || i % 4 ==1){
                for(int j = 0; j < col; j++){
                    if(j % 4 == 0 || j % 4 ==1){
                        array[i][j] = 1;
                    }
                }
            }else{
                for(int j = 0; j < col; j++){
                    if(j % 4 == 2 || j % 4 == 3){
                        array[i][j] = 1;
                    }
                }
            }
        }
        
        int count = 0;
        for(int i = 0; i < row; i++){
            for(int j = 0; j < col; j++){
                if(array[i][j] == 1){
                    count++;
                }
            }
        }
        System.out.println(count);
    }
}